Bistable switch with controlled refiring threshold



June 6, 1967 D. A. ZELLER, JR

BISTABLE SWITCH WITH CONTROLLED REFIRING THRESHOLD Filed July 17, 1963PRIOR ART 5 V/ hm w A w w A r E L 7 V E W Z A A m V atent @fifice3,3243% Patented June 6, 1967 3,324,309 BISTABLE SWITCH WITH CGNTRGLLEDREFERING THRESHOLD David A. Zeiier, J12, Brookfield, Conn, assignor toData- (Iontrol Systems, Inc Danbury, (loam, a corporation of DelawareFiled July 17, 1963, Ser. No. 295,743 8 Claims. (Cl. 307-885) Thisinvention applies in general to bistable triggers and more particularlyto an improvement in Schmitt triggers which will permit designing asymmetrically firing Schmitt trigger.

Schmitt triggers, which are fast switching bistable triggers, are wellknown in the art. Their attributes and deficiencies have been frequentlydiscussed in the literature. Probably the major problem in using aSchmitt trigger is the problem to which this invention is addressed.That problem resides in the fact that the critical voltage at which aSchmitt trigger will fire, in one direction, as the input signal to thetrigger is increasing is difierent than the critical voltage at whichthat trigger will refire as the input signal is decreasing.

It is a major purpose of this invention to provide an improvement to theSchmitt trigger so that the voltage at which triggering occurs on theascending slope of an input signal will be the same as the voltage atwhich triggering occurs on the descending slope of an input signal.

The difference in firing voltage at which the Schmitt trigger fires andrefires is known as hysteresis. It is widely recognized that hysteresisis desirable in order to provide stability of switching and in order tospeed up the rate at which switching occurs. Since the Schmitt triggeris designed for fast, stable switching, the loss of hysteresis is mostundesirable.

Accordingly, it is a more specific purpose of this invention to providea compensation for hysteresis that will result in symmetrical triggeringwithout doing away with the hysteresis so that stability of operationand fast switching can be coupled with symmetrical triggering.

Prior art designs which are provided for symmetrical triggering havedone so by eliminating the hysteresis and thus have resulted in circuitsof limited stability. Before this invention, it had been presumed thathysteresis and symmetrical firing were conflicting characteristics. Thisinvention teaches a technique whereby one may design to obtain bothhysteresis and symmetrical firing.

Broadly and briefly speaking, the design technique taught by thisinvention involves a means for shifting the voltage level of thehysteresis after switching has occurred by an amount equal to thehysteresis. Thus full advantage is taken of the hysteresis effect toassure fast and stable switching. Then after the switching has beencompleted, the biasing level in the trig er is shifted by an amount thatcompensates for the hysteresis and permits refiring at the same voltagelevel as the firing level.

Other objects and purposes of this invention will become apparent from aconsideration of the following detailed description and drawings, inwhich:

FIG. 1 is an electrical schematic of a transistorized prior art Schmitttrigger, and

FIG. 2 is an electrical schematic of a Schmitt trigger incorporating oneembodiment of the improvement taught by this invention.

Concerning terminology Throughout this application, it should beunderstood that a reference to an increase in voltage or a decrease involtage does not refer to absolute magnitude but takes into account thesign of the voltage involved. Thus a change in potential from minus 12volts to zero volts would be considered an increase in voltage. If thisvoltage were applied to the base of a transistor, it might be referredto herein as an increase in bias. Consistent with this terminology, whenit is said herein that a decrease in potential on the collector of thetransistor Q results in pulling down the base of the transistor Q it ismeant that the base of the transistor Q goes from a more positive to aless positive value. To say that the base of the transistor Q goes froma more positive to a less positive value may include passing throughground as when going from a positive value to a negative value. Indeed,consistent with this invention, a point that goes from let us say minustwo volts to let us say minus 10 volts would be considered as going froma more positive to a less positive value.

Reference is made throughout this application to a Schmitt triggerbecause that term has achieved widespread use and covers a fairly clearcut class of trigger designs. However, it should be understood that thisininvention has applicability to any bistable switching circuit whichexhibits the hysteresis characteristic above described.

The prior art FIG. 1 illustrates a typical Schmitt trigger without theimprovement added by this invention. The Schmitt trigger shown happensto involve two NPN transistors Q and Q2.

The Schmitt trigger is a binary circuit having two states, in one statethe transistor Q is on and the transistor Q is off; while in the otherstate the transistor Q is on and the transistor Q is off. The Schmitttrigger has the distinction over other binanry circuits of having a veryfast switching action, that is a very short switching time. The tying ofthe two emitters together and the provision of a common emitter resistorR provides the additional regenerative feedback which speeds up theswitching action. Since the operation of the Schmitt trigger is wellknown to those skilled in this art, it will not be necessary to do morethan outline its operation to set the base for a discussion of theimprovement provided by this invention. A more comprehensive discussionof the operation of the Schmitt trigger may be had by reference toparagraphs 510 and 5-11 in Pulse and Digital Circuits by Millman andTaub (McGraw-Hill, 1956). The Millman and Taub reference discusses theoperation of the Schmitt trigger when designed to employ electron tubesinstead of transistors. A short explanation of a transistorized Schmitttrigger is provided in paragraph 196 in Basic Theory and Application ofTransistors (Army Technical Manual TM11-690, 1959).

Suffice it to say here, that the resistors R and R operate as a voltagedivider to provide a control voltage on the base of the transistor Q Thetrigger of FIG. 1 is designed so that when in a quiescent state (that isthere being no input signal E the transistor Q will be shut ofif becauseits base is connected to the negative supply B- through the resistor RUnder these conditions the transistor Q will conduct since its emitteris negative, being connected to the negative supply B through theresistor R and its base is sufliciently positive relative to its emitterbecause of the selection of appropriate values for the voltage dividerresistors R and R In the design illustrated herein (with the resistorand power supply values indicated'in the figures), the base of thetransistor Q will be at approximately zero potential when transistor Qis cut off. Since transistor Q is conducting, its emitter will followits base and the emitter current will be whatever is required to providethe required drop across the emitter resistor R The design is such thatthe emitter current required will assure conduction in the saturationportion of the Q characteristic. In brief, Q is hard on.

The input signal E when applied to the base of the transistor Q willturn on that transistor Q at the moment when the signal E becomessufiiciently positive to provide the forward bias necessary to cause thetransistor Q to start conducting.

As the transistor Q turns on in response to the appropriately positivesignal on its base, current through the resistor R will increase therebycausing a drop in potential at the collector of the transistor Q whichdrop in potential will be reflected in a proportionate drop in potentialat the base of the transistor Q The proportionate voltage drop at thebase of the transistor Q is a function of the ratio between the voltagedivider resistors R and R Thus as the potential on the base of Q goesnegative there is a tendency for the transistor Q to turn off.

To speed up the switching there is included in the Schmitt trigger typeof design a connection between the emitter of the transistor Q and theemitter of the transistor Q which connection is then connected to thenegative supply B through the resistor R As the transistor Q turns on,the increased current through the resistor R results in a less negativeemitter potential (on both the emitters of the transistors Q and theemitter of the transistor Q Thus the flow of current through thetransistor Q as that transistor turns on, simultaneously reduces thevoltage on the base of the transistor Q and increases the potential onthe emitter of the transistor Q In this fashion the time it takes toturn olf the transistor Q is relatively small and the switching can beconsidered to be relatively fast as compared to other bistable designs.

The resistor values, voltages and designs are such that the transistorsare turned hard on and hard off so that they operate in their saturationregion and the magnitude of transistor current is substantiallyindependent of the magnitude or rise time of the input signal E When theinput signal E decreases in voltage magnitude below a certain criticalpoint, this bistable trigger will switch back the other way. As theinput signal E decreases, the base of the transistor Q is renderedsufiiciently negative by the negative supply B so as to turn thetransistor Q off. As it starts to turn off, a decrease in currentthrough the transistor Q will result in less current through theresistor R so that the collector of the transistor Q will go morepositive and, accordingly, a proportionate increase in potential will beapplied to the base of the transistor Q Concurrently, the decrease incurrent through the common emitter resistor R will drop the potential onthe emitters therefore increasing the forward bias on the transistor Qand increasing the rate at which the transistor Q will turn on.

This Schmitt trigger, as does all Schmitt triggers, exhibits the wellknown Schmitt trigger phenomenon that is called hysteresis. This meansthat the magnitude of the critical signal voltage to trigger thetransistor Q on is greater than the magnitude of the critical voltagenecessary to switch the transistor Q off. This phenomenon may beappreciated by assuming that the transistor Q, is turned on by, let ussay, an increase in base voltage from just below the firing point tojust above the firing point. When this occurs, the collector of thetransistor Q drops in potential because of the increased current flowthrough the transistor Q and, correspondingly, the base potential on atransistor Q decreases a proportional amount. In addition, once thetransistor Q has been turned on, its emitter will be at substantiallythe same potential as its base and, since the emitters of thetransistors Q and Q are tied together, the emitter of the transistor Qwill follow the emitter of the transistor Q and go more positive. Thusthe base of the transistor Q will, on the switching action, drop anumber of volts negative and the emitter of the transistor Q will rise anumber of volts so that the transistor Q will be. turned hard off. Inthis fashion, a very small increment in transistor Q base voltage fromjust below its triggering point to just above its triggering point willresult in a great change in base to emitter voltage in the transistor QAccordingly, a small drop in the base voltage on the transistor Q fromjust above its firing point to the voltage that it had just before itfired will not suffice to turn on the transistor Q nor turn off thetransistor Q Thus the base voltage on the transistor Q must drop wellbelow its on firing point in order to cause the reverse triggering. Thisdifference between the triggering voltage necessary on the ascendingslope of an input signal E and the descending slope of an input signal Ehas been called hysteresis.

It is this hysteresis which produces stability in the Schmitt triggersince it assures that a switch once it starts will continue. Thehysteresis is also a concomitant of the fast switching.

The invention FIG. 2 is an embodiment of this invention and specificallyillustrates an improvement on the prior art Schmitt trigger which willpermit setting the refire point at the same voltage level as the firingpoint while retaining the benefit of the hysteresis.

FIG. 2 illustrates a symmetrically firing Schmitt trigger which fires onthe ascending slope of the input signal E in exactly the same fashion asdoes the prior art trigger illustrated in FIG. 1. However, once it hasfired, so that the transistor Q is on (and the transistor Q is off) acurrent is pumped through the resistor R in such a direction andmagnitude so as to change the potential on the base of the transistor Qin a direction and to an extent sufiicient to cause the transistor Q torefire once the input signal has decreased to a magnitude substantiallyequal to the triggering voltage. This result is achieved by use of thecurrent source Q Before describing the operation of the current source Qit should be noted that the single Q collector resistor R of the priorart design has been split into two resistors R and R so that the base ofthe current source Q may be connected between the resistor R and theresistor R In this fashion the voltage magnitude 0n the base of thecurrent source Q will change proportionately with the voltage changes onthe collector of the transistor Q In addition, the Q base resistor R ofthe prior art is split into two resistor R and R so that the collectorof the current source Q can be connected, as shown, between these tworesistors R and R In this fashion, the voltage on the base of thetransistor Q will be modified as a partial function of the magnitude ofthe current that is supplied by the current source Q It should be notedthat the current source Q in the embodiment shown is a PNP transistor.Accordingly, Q will tend to be turned on by a drop in its base bias. Thecircuit is designed so that an appropriate drop does occur when thecollector Q drops as Q is turned on. The base bias drop in Q turns on Qso that a current will flow through Q and the resistor R The consequentcurrent flow in the resistor R raises the base bias on this NPNtransistor Q to a point where the transistor Q can be more readilyturned on than just prior to the Q current flow. The triggering voltageas the input signal E descends can thus be set to be equal to thetriggering voltage when E increases so that the result is asymmetrically firing Schmitt trigger.

The switching action between Q and Q occurs very much more rapidly thanthe turning on the transistor Q;,. As a consequence, the increase in thebias on the base of the transistor Q due to Q current flow occurs afterthe bias on the base of Q has been decreased by the switching actionbetween the transistors Q and Q If the time lag between the switching ofthe transistors Q and Q and the turning on of the current source Q; isnot great enough, a capacitor C may be added to increase the time lagsufficiently so that there will be no interruption with the fastswitching operation and so that the switching operation will not berendered unstable.

The magnitudes of the resistors R and R are selected to control themagnitude of the current provided by the current source Q Because ofthis increased current through the resistor R the base to emittervoltage of the transistor Q is modified in such a fashion that thetransistor Q will turn on when the signal voltage E decreases to thedesired switching voltage. It should be pointed out that this reverseswitching occurs because as E decreases, the emitter of the transistor Qalso decreases since it is tied to the base of the transistor Q when thetransistor Q is turned on. Since the emitter of Q decreases, the emitterof Q must also decrease, it being tied directly to the emitter of thetransistor Q When the emitter of Q decreases to the point such thatthere is sufificient forward bias on the transistor Q the transistor Qwill turn on. As Q turns on, Q turns ofi due to the decreased basevoltage on Q resulting from the decreasing input signal E and due to theincreasing emitter voltage resulting from an increased current throughthe common emitter resistor R The turning off of the transistor Qconcurrently causes a decrease in current through the collectorresistors R and R so that the collector of the transistor Q rises,resulting in an increase in the base voltage of the transistor Q Thisfeedback turns the transistor Q on even harder so that a bistableswitching operation occurs. Concurrently, the decrease in currentthrough the collector resistors R and R causes an increase in biasvoltage on the PNP transistor Q so that the current source Q tends toturn off resulting in a decrease in the current through the resistor RThis decrease in current through the resistor R returns the base voltageof the transistor Q to its original value so that there will berepeatable switching when the input signal E starts to rise again.

From the above description it should be clear that in brief terms thisinvention involves the provision of a current source whose magnitude isresponsive to the collector voltage on the transistor Q and this currentwill be used to modify the base voltage on the transistor Q A currentsource having these characteristics and operating (in response tochanges in the collector voltage on the transistor Q) at a slower ratethan the switching rate will provide the change in transistor Q basevoltage necessary to compensate for the hysteresis and yet will notcancel out the hysteresis while switching takes place. Thus thisinvention retains the advantages of having hysteresis during switchingand compensating for the effect of hysteresis during refiring.

The above description is of one particular embodiment of this invention.It would be apparent to those skilled in this art to make suchmodifications as would be necessary to adapt this invention to valvesother than the NPN transistors Q and Q illustrated. Thus, PNPtransistors could be employed with the appropriate change in voltagesigns. It would also be possible to apply this invention to a Schmitttrigger employing electron tubes. Accordingly, it is intended in theclaims to cover all such modifications as would fall within the scope ofthe invention.

For example, the major intended use of the invention is to design asymmetrically firing Schmitt trigger. However, there is no reason whythe invention should be limited to those applications where the refiringpoint must be the same as the firing point. In effect, this inventionprovides a technique whereby the amount of hysteresis can be controlledby the design explained herein. The amount of hysteresis compensationcan be designed to be any predetermined magnitude. Where designed for asymmetrical ly firing Schmitt trigger, the compensation is sufiicient toequal the hysteresis and thus cancel out its net efiect on refirevoltage. In other applications differing amounts of compensation can beused so as to set the refiring point at any level.

The amount of hysteresis compensation supplied can be sufiiciently greatso as to reverse the relative voltage levels between the fire and refirepoints. For example, in the embodiment illustrated, the refire point isat a lower voltage level than the firing point. By means of thisinvention, the hysteresis could be so over-compensated that the refirepoint would be at a higher voltage than the firing point. Thisover-compensation of hysteresis would not otherwise affect the operationof the switch in that the switch would still fire on a rising signal andrefire on a falling signal.

This ability to change the relative voltage levels of the fire andrefire point is unique with this invention and cannot be achieved withthe hysteresis compensation techniques employed in prior art Schmitttriggers.

In addition to the particular type of design illustrated above in whicha signal is supplied that tends to cancel hysteresis and that, by theproper selection of parameters, either reduces hysteresis, cancels thehysteresis or overcompensates for the hysteresis, it is possible tosupply a signal which will tend to add to the hysteresis alreadypresent. Thus it is within the ambit of this invention to supply a biasto the control signal of the second valve (that is to the base of thetransistor Q in this embodiment) which will add to, as contrasted withcompensate for, the hysteresis otherwise developed by the switchingdesign.

The switch illustrated is one which fires on a rising input signal andrefires on a declining input signal. However, a switch can readily bedesigned to fire on a declining input signal and refire on a risinginput signal and the teachings of this invention can be applied to sucha switch to cause a modification in the hysteresis inherent in thatswitch design.

Concerning terminology in the following claims, it should be understoodthat the use of the term function to relate parameters refers to partialfunction as well as complete function. Thus the bias on the transistor Qis a function of the state of the transistor Q because the state of Qdoes affect that bias even though other factors such as the bus barvoltage levels, the magnitudes of the resistors R and R and the currentoutput from the transistor Q also affect the base bias on Q Accordingly,the base bias on Q is only a partial function of the state of Q yet,herein, it is to be understood that the term function includes partialfunction.

The embodiment described in detail above broadly involves a controlmeans Q which responds to the state of the transistor Q to modify thebias on the transistor Q The control means could just as Well bedesigned to sense the state of the transistor Q in order to modify thebias on the transistor Q and in that fashion to modify the hysteresis ofthe overall switching device. After all, this is a bistable device andthe modification of the bias on either of the switching elements wouldhave comparable results. It is to be understood, herein, that areference to a first valve and a second valve in the claims is strictlyfor the purpose of subsequent reference within each claim and the phrasefirst valve, in general, refers to whichever valve is the one whosestate is sensed by the control means. The fact that the transistor Q isthe valve at the input to the circuit does not mean for the purpose ofthis application and its claims that it is necessarily a first valve.

What is claimed is:

1. In a bistable switching circuit having a first valve and a secondvalve, each of said valves having a first state and a second state andbeing interconnected such that said circuit has a first stable state anda second stable state, each of said valves having a control element anda low impedance output element, the low impedance elements of saidvalves being coupled to one another and having a common resistor, saidcircuit including an input lead connected to the control element of saidfirst valve so that an input signal passing a first threshold voltage ina first direction will fire said circuit into its first state 7 and aninput signal passing a second threshold voltage in a second directionwill refire said circuit into its second state, the improvementcomprising:

a voltage dividing bias means coupled to the control element of saidsecond valve, and

control means responsive to said second state of said circuit to providea control signal when said circuit is in said second state, said controlsignal being coupled to a point on said voltage dividing bias means tomodify the bias on said control element of said second valve during saidsecond state of said circuit.

2. The bistable switching circuit improvement of claim 1 wherein saidcontrol means is a current source comprising a third valve having an offstate and an on state, the output of said current source being coupledto said point on said voltage dividing bias means, the control elementof said third valve being coupled to the output of said first valve sothat the state of said first valve will determine the state of saidthird valve.

3. The bistable switching circuit of claim 1 wherein said voltagedividing bias means is a first and second resistor connected in seriesto said control element of said second valve, and wherein the juncturebetween Said resistors is said point to which said control signal iscoupled.

4. The bistable switching circuit improvement of claim 1 furthercharacterized by:

time delay means connected to said point on said voltage dividing biasmeans to provide a predetermined time delay in the application of saidcontrol signal to said control element of said second valve.

5. In a bistable switching circuit having a first transistor and asecond transistor, each of said transistors having base, collector andemitter electrodes, said emitter electrodes being coupled to one anotherand having a common emitter resistor, said transistors being coupled toone another to provide said circuit with a first stable state and asecond stable state, said circuit including an input lead connected tosaid base of said first transistor so that a rising input signal passinga first threshold voltage will fire said circuit into said first stablestate and a falling input signal passing a second threshold voltage willrefire said trigger into said stable state, the improvement comprising:

a voltage dividing bias means coupled to said base of said secondtransistor, and

a current source responsive to said second state of said circuit toprovide a control signal when said circuit is in said second state, saidcontrol signal being connected to a point on said voltage dividing biasmeans 5 to modify the bias on said base of said second transistor duringsaid second state of said circuit, whereby said bias on said base ofsaid second transistor will have a first predetermined value when saidcircuit is in said first state and a second predetermined value whensaid circuit is in said second state. 6. The bistable switching circuitimprovement of claim 5 wherein said current source is a third transistorhaving an off state and an on state, said third transistor having base,collector and emitter electrodes, said base electrode of said thirdtransistor being coupled to said collector of said first transistor sothat the state of said first transistor will determine the state of saidthird transistor, the current output of said third transistor being saidcontrol signal that is connected to said point on said bias means.

7. The bistable switching circuit of claim 5 wherein said voltagedividing bias means is a first resistor and a second resistor connectedin series to said base of said second transistor, and wherein thejuncture between said resistors is said point to which said controlsignal is connected.

8. The bistable switching circuit improvement of claim 7 furthercharacterized by:

a capacitor connected across said second resistor to provide apredetermined time delay in the application of said control signal tosaid base of said second transistor.

References Cited UNITED STATES PATENTS JOHN w. HUCKERY, PrimaryExaminer.

J. D. CRAIG, Assistant Examiner.

1. IN A BISTABLE SWITCHING CIRCUIT HAVING A FIRST VALVE AND A SECONDVALVE, EACH OF SAID VALVES HAVING A FIRST STATE AND A SECOND STATE ANDBEING INTERCONNECTED SUCH THAT SAID CIRCUIT HAS A FIRST STABLE STATE ANDA SECOND STABLE STATE, EACH OF SAID VALVES HAVING A CONTROL ELEMENT ANDA LOW IMPEDANCE OUTPUT ELEMENT, THE LOW IMPEDANCE ELEMENTS OF SAIDVALVES BEING COUPLED TO ONE ANOTHER AND HAVING A COMMON RESISTOR, SAIDCIRCUIT INCLUDING AN INPUT LEAD CONNECTED TO THE CONTROL ELEMENT OF SAIDFIRST VALVE SO THAT AN INPUT SIGNAL PASSING A FIRST THRESHOLD VOLTAGE INA FIRST DIRECTION WILL FIRE SAID CIRCUIT INTO ITS FIRST STATE AND ANINPUT SIGNAL PASSING A SECOND THRESHOLD VOLTAGE IN A SECOND DIRECTIONWILL REFIRE SAID CIRCUIT INTO ITS SECOND STATE, THE IMPROVEMENTCOMPRISING: A VOLTAGE DIVIDING BIAS MEANS COUPLED TO THE CONTROL ELEMENTOF SAID SECOND VALVE, AND